One of the major problems faced by wireless telecommunications providers is the progressive loss of voice quality as voice data passes through multiple stages of vocoding (voice encoding and decoding). One such example is when a call is placed from one mobile phone to another using today's so-called “second generation” cellular equipment. If the two subscribers are connected to different wireless base station equipment, where the two sets of base station equipment are connected to each other via a Public Switched Telephone Network (PSTN), voice data goes through two encoders and two decoders. It is first encoded at the originating wireless terminal (for example a mobile station). It is then decoded by the base station equipment that the originating wireless terminal is connected to, before passing over the PSTN as pulse code modulated data. It is once again encoded by the base station equipment that the destination wireless terminal is connected to, before being sent over the air. Finally, it is decoded by the destination wireless terminal. There is serious loss of voice quality associated with the successive encoding and decoding of the voice data.
This problem is being addressed by designers of next-generation cellular systems. Two approaches have been proposed. The first approach is to modify the behavior of the two vocoders in the base station equipment (the second and third vocoders in the above example, also known as the tandemed vocoders) in order to minimize the distortion that is introduced. This approach is described in U.S. Pat. No. 5,995,923, entitled “Method and Apparatus for Improving the Voice Quality of Tandemed Vocoders,” whose contents are incorporated herein by reference.
Another approach is to use signaling between the two sets of base station equipment in order to bypass the tandemed vocoders completely. This second approach is known as “tandem free operation” and is described in international patent application serial number PCT/CA95/00704 dated Dec. 13, 1995, U.S. Pat. No. 6,172,974 entitled “Network Element Having Tandem Free Operation Capabilities,” and U.S. Pat. No. 6,272,358 entitled “Vocoder By-pass for Digital Mobile-to-mobile Calls.” The contents of the three references mentioned in the previous sentence are incorporated herein by reference. One implementation of this approach is given in [3GPP2] listed in the REFERENCES section of this application.